local anaesthesia Flashcards
describe the generation of APs
- depolarising stimulus: Na channels open, Na enters cell
- inactivation: Na channels close, K channels open so K leaves cell
- cell refractory state: Na channels restored to resting state but K channels still open so cell is refractory
- resting state: Na and K channels restored to resting state
descrube the structure of LAs
- aromatic region: very lipid soluble/hydrophobic
- amine side chain: hydrophilic
- ester or amide bond (cocaine - ester, lidocaine - amide)
which LA doesn’t obey the above structure law?
- benzocaine
- no basic amine group so weaker potency
what is the hydrophilic pathway in MoA on VGSCs?
- drug remains in eqm between ionized and unionized forms
- all LAs are weak bases
- unionised form can pass across membrane but cannot have any action
- ionised form is needed to have action but cannot pass across membranes
- pathway is use-dependent as channels need to be open for cation drug to access VGSCs
what is the hydrophobic pathway in MoA on VGSCs?
- lipid soluble drugs can access hydrophobic pathway
- drop into channel when channel is closed
- not use-dependent
what are the effects of LAs?
- prevent generation and conduction of APs
- don’t inflence resting potentials
- influence channel gating (e.g. hold in an inactivated state)
- influence surface tension (lower)
- selectively block small diameter fibres e.g. nociceptive pain fibres
- selectively block non-myelinated fibres
describe the chemical properties of LAs
- weak bases
- pKa 8-9
- mostly ionised and so less pass into axons of neurones
- as they have a high pKa, they are use-pH-dependent
what does this mean in infected tissues?
infected tissues are normally slightly acidic so LA is less effective as more will be ionised
what is the route of admin of surface anaesthesia? what is it used for?
- spray/powder form
- mucosal surfaces e.g. mouth, bronchial tree
- high conc can lead to systemic toxicity
- sore throat relief
what is the RoA of infiltration anaesthesia? What is it used for?
- SC injection
- used in minor surgeries
- used for post-surgery sutra LA analgesia
what is co-administered with the LA?
- adrenaline
- vasoconstrict
- slows down diffusion of LA away from site of injection
- lower conc of LA needed
- reduces systemic toxicity
what is the RoA of IV regional anaesthesia? What is it used for?
- IV injection distal to pressure cuff
- used in limb surgery
- systemic toxicity if cuff is released prematurely
- used in trigger finger repair
what is the RoA of nerve block anaesthesia? What is it used for?
- injection close to nerve trunk
- e.g. dental nerves
- widely used
- low doses and slow onset of action
- vasoconstrictor co-admin often
what is the RoA of spinal anaesthesia? what is it used for?
- intrathecal (sub-arachnoid space injection)
- injection close to spinal roots e.g. lower limb surgery
- reduces BP so can cause prolonged headache
- hip replacement
what can be added with the LA in spinal anaethesia?
glucose
inc. specific gravity so LA doesn’t travel up to CSF to brain
